Nebula- and Hugo-winner Kress mixes time travel, global catastrophe, and mysterious aliens in this strong postapocalyptic tale. Gruff teenager Pete makes brief runs back in time from the desolation of the year 2035, using alien technology that can only transport children, to pick up fresh supplies and recruits. Julie, a mathematician in 2013, finds a pattern in a series of kidnappings. They are, of course, Pete’s expeditions, and as the two head towards their inevitable collision, the clock ticks down on the catastrophe that will turn Julie’s orderly world into Pete’s devastated landscape. Kress handles the crisscrossing timelines with cool elegance, making a complicated story clean and easy to read. Some readers may find both the setting and the characters a little severe, but the ultimate message of hope and understanding resonates strongly. (May)